The present invention relates generally to semiconductor-on-insulator (SOI) devices and methods of forming the same and, more particularly, to SOI devices and methods for forming which avoid or reduce floating body effects using a silicon-germanium/silicon heterojunction between the source/drain and the body of a transistor.
Traditional semiconductor-on-insulator (SOI) integrated circuits typically have a silicon substrate having a buried oxide (BOX) layer disposed thereon. A silicon active layer is disposed on the BOX layer. Within the active layer, active devices, such as transistors, are formed in active regions. The size and placement of the active regions are defined by shallow trench isolation (STI) regions. As a result of this arrangement, the active devices are isolated from the substrate by the BOX layer. In addition, a body region of each SOI transistor does not have body contacts and is therefore xe2x80x9cfloating.xe2x80x9d
SOI chips offer potential advantages over bulk chips for the fabrication of high performance integrated circuits for digital circuitry. Such digital circuitry is typically made from partially-depleted metal oxide semiconductor field effect transistors (MOSFETs). In such circuits, dielectric isolation and reduction of parasitic capacitance improve circuit performance, and virtually eliminate latch-up in CMOS circuits. In addition, circuit layout in SOI can be greatly simplified and the packing density greatly increased since.
However, MOSFETs on SOI materials are known to exhibit parasitic effects due to the presence of the floating body. These effects are known as floating body effects and may result in undesirable performance in SOI devices. It will be appreciated from the foregoing that a need exists for SOI MOSFETs having reduced floating body effects.
According to one aspect of the invention, the invention is a semiconductor-on-insulator (SOI) device. The SOI includes a substrate having a buried oxide layer disposed thereon; and an active layer disposed on the buried oxide layer, the active layer having an active region defined by isolation regions, the active region having a source and a drain with a body disposed therebetween. Each of the source and the drain have a silicon extension, each extension respectively forming a portion of a source/body junction and a drain/body junction; and a selectively grown silicon-germanium region disposed under an upper layer of selectively grown silicon, the silicon-germanium regions forming heterojunction portions respectively along the source/body junction and the drain/body junction, wherein an upper surface of each silicon-germanium region has a height equal to or lower than a junction depth of the source and drain extensions.
According to another aspect of the invention, the invention is a semiconductor-on-insulator (SOI) device. The SOI device includes a substrate having a buried oxide layer disposed thereon; and an active layer disposed on the buried oxide layer, the active layer having an active region defined by isolation regions, the active region having a source and a drain with a body disposed therebetween. One of the source or the drain has a silicon extension, the extension respectively forming a portion of a junction with the body; and a selectively grown silicon-germanium region disposed under an upper layer of selectively grown silicon, the silicon-germanium region forming a heterojunction portion along the junction with the body, wherein an upper surface of the silicon-germanium region has a height equal to or lower than a junction depth of the extension.